After upgrade or most likely after a migration search in outlook 2011/2016/2019 will stop working.
This is could be because the spotlight search index is broken or missing. 8 out of 10 times is helps that rebuild the spotlight index.
Do like this: Go to system preferences/Sptlight and choose integrity. Add you HD to prevent spotlight to search. Choose something else in system P and then go back to Spotlight and remove the HD. This causes Spotlight to rebuild all indexes for the whole HD.
This can take a while.........
If after a few hours search is still not working try this. Start your terminal. It is in Applications/utilities.
In terminal write this: mdimport -L
Do not close the terminal but try the search in outlook now. Still no results? Follow this link or call an expert.